Modelling land-use sustainability using farmland birds as indicators

Biodiversity on farmland is declining due to agricultural intensification and occurs across many taxa such as plants, insects or birds. Here, we modelled population sizes of five farmland birds in central Germany as the German Sustainability Indicator for Species Diversity (SISD) is based on this taxon. We explored options for sustainable farmland management by generating land-use scenarios at the regional scale. For individual bird species, high SISD scores could be reached by changing environmental variables, such as landscape or crop diversity, percent cover of spring cereals or hedge density. However, contrasting species responses to these variables prevented from reaching high scores for all species simultaneously. We were able to improve the total SISD score from 0.77 at present to 0.94 by increasing landscape and crop diversity or to 0.87 by increasing hedge density and reducing spring cereals, respectively. An economic evaluation of the return losses associated with these changes revealed that annual costs of approx. 5.5 €/ha farmland would suffice for this latter increase by optimizing hedges and spring cereals towards high SISD. We conclude that balancing three levels of trade-offs, i.e. contrasting requirements of species, diverging responses in different landscapes, and alternative economic options, is difficult to achieve without systematic modelling. Yet, by accounting for differences in landscape structure and species distributions at a regional spatial scale and by focusing on clearly defined measures such as hedge density or the cover of spring cereals rather than composite indices like landscape or crop diversity it seems possible to develop realisable, affordable and sustainable management strategies.